Recent progress in miniaturization and integration of LSI chips has raised an issue of increase in the amount of heat generated from LSI chips. As a countermeasure, research and development have been made on lowering power supply voltages to reduce the heat amount due to large scale integration. However, in a circuit that operates at a low voltage, the threshold voltage of transistors is set low and a leakage current is likely to increase. Reducing the heat amount by lowering power supply voltages now faces limits. Under these circumstances, there is a demand for a low-cost cooling system that can cool a LSI chip in an efficient manner, which chip is a high-density heat source equivalent to a nuclear reactor of 100 W/cm2. Such a low-cost cooling system is used to cool not only signal-processing semiconductor devices such as CPUs, but also semiconductor lasers and illumination-purpose light-emitting diodes.
A water cooling system is expected as a high efficiency cooling system that is to be substituted for a conventional cooling fan. In a water cooling system, a refrigerant or a coolant flows through a sealed heat sink called a “water jacket” (or water pillow). A pump may be used to circulate the refrigerant. However, the technology has been slow to adopt a pump in a cooling system because a pump is an extra component in terms of cost and power consumption. In addition, when a water pillow is made smaller to fabricate a high-efficiency water jacket, pressure loss will increase in a water channel and the power-consumption and the mechanical workload on the pump will increase. The larger the pump size, the greater the cost is. Increase in the power consumption also increases the amount of heat dissipation.
One possible method is to use an auxiliary pump. Conventionally, piezoelectric diaphragm pumps have been suggested (see, for example, Patent Document 1 listed below). FIG. 1 illustrates a structure of a conventional micropump using a piezoelectric diaphragm. The pump has a diaphragm 1010 consisting of a piezoelectric plate facing a pressure chamber 1050. An upper electrode 1011 and a lower electrode 1013 of the piezoelectric plate (diaphragm) 1010 are connected to an upper wiring 1021 and a lower wiring 1023, respectively, to cause the diaphragm to move in the vertical direction under the application of a voltage. In this configuration, power is supplied to drive the pump to perform mechanical work. However, the voltage level for driving a piezoelectric actuator is generally high, requiring electric energy to operate the driving circuit. In addition, it is difficult for the piezoelectric actuator to increase the flow rate of the pump due to small displacement.
On the other hand, a switch using a temperature-sensitive magnetic material with a Curie point is known (see, for example, Patent Document 2 and Patent Document 3 listed below). By heating a stationary temperature-sensitive magnetic material to a temperature above the Curie point, a change is caused in the magnetic field, which phenomenon serves as a switch. However, once a temperature becomes high, the switch always remains in the ON state without performing ON/OFF switching.